<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div{
            border: 1px solid #000;
        }
        #mydiv{
            width: 800px;
            height: 500px;
            display: flex;
            flex-direction: column;
            justify-content: center;
            align-items: center;
        }
        #mydiv div{
            width: 100px;
            height: 100px;
            background-color: red;
        }
    </style>
</head>
<body>
    <div id="mydiv">
        <div></div>
        <div></div>
        <div></div>
        <div></div>
    </div>
</body>
</html>